1. Field of the Invention
The present invention relates to a lithographic system, and more particularly to a manner of control of the illumination system in a lithographic apparatus.
2. Description of Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection systems, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The illumination system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam of radiation, and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures. Twin stage lithographic apparatuses are described in International Patent Applications WO 98/28665 and WO 98/40791, for example.
Lithographic projection apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed v at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
In a lithographic apparatus, the size of features that can be imaged onto the wafer is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatuses employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use higher frequency (energy) radiation, e.g. EUV or X-rays, or particle beams, e.g. electrons or ions, as the illumination radiation in lithographic apparatuses.
However, the glass or quartz plates on which a conventional reticle pattern is defined are generally not transparent to some of these forms of illumination radiation. As an alternative in the case of charged-particle lithography, for example, the reticle is formed of a material, e.g. metal, that is opaque to the form of radiation used and in which apertures are cut to define the reticle pattern. To avoid the need to provide obscuring support arms to opaque islands in the pattern, the reticle pattern is divided into a plurality of sub-patterns separated by supporting struts. The complete pattern is correctly imaged on the wafer by introducing successive shifts in the illumination beam after it has passed through each sub-pattern. This type of arrangement is sometimes referred to as a xe2x80x9cstrutted maskxe2x80x9d and an example is disclosed in U.S. Pat. No. 5,079,112.
General information with regard to the use of electron beams in lithography can be gleaned, for example, from U.S. Pat. No. 5,260,151.
As disclosed in EIPBN, May 1998 AE6, xe2x80x9cCritical dimension control at stitched sub-field boundaries in a high-throughput SCALPEL systemxe2x80x9d, if the beam intensity profile of the illumination radiation is rectangular, then any positional inaccuracy in the stitching process will result in a substantial dose error. Such a stitching procedure using rectangular beam profiles is sometimes referred to as xe2x80x9csimply-buttedxe2x80x9d and if the beam positions are not accurate there will be a region of no dose or double dose along the stitch seams. The EIPBN article therefore proposes the use of a trapezoidal beam profile and deliberate overlap in the stitching process. Positional inaccuracies then result in smaller dose errors.
The EIPBN article does not, however, disclose any method for generating an illumination beam having the desired trapezoidal intensity profile.
An object of the present invention is to provide a convenient means of generating an illumination beam having a trapezoidal intensity profile in a lithographic projection apparatus.
According to the present invention there is provided a lithographic projection apparatus for imaging of a mask pattern in a mask onto a substrate provided with a radiation sensitive layer, the mask having a plurality of transmissive regions bounded by opaque regions. The lithographic projection apparatus comprises: a radiation system comprising a radiation source and an illumination system for generating an illumination beam; a first movable object table provided with a mask holder for holding a mask; a second movable object table provided with a substrate holder for holding a substrate; and a projection system for imaging irradiated portions of the mask onto target portions of the substrate. The illumination system changes the size or position of the illumination beam on the mask during the course of an exposure period of at least part of a given transmissive region so as to generate an effective trapezoidal beam profile.
The term xe2x80x9ctransmissive regionxe2x80x9d is intended to refer to a region of the mask which is at least substantially transparent to the illumination radiation used.
With the present invention it is possible to easily generate the desired beam profile using electronic control of the beam size or position.
According to a yet further aspect of the invention there is provided a method of manufacturing a device using a lithographic projection apparatus comprising a radiation system comprising a radiation source and an illumination system for generating an illumination beam; a first movable object table provided with a mask holder for holding a mask; a second movable object table provided with a substrate holder for holding a projection system for imaging irradiated portions of the mask onto target portions of the substrate provided with a radiation-sensitive layer, so as to partially overlap. The size or position of the illumination beam on the mask is changed during the course of an exposure period of at least part of a given transmissive region so as to generate an effective trapezoidal beam profile.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallisation, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmasks, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.
In the present document, the terms illumination radiation and illumination beam are used to encompass all types of electromagnetic radiation or particle flux, including, but not limited to, ultraviolet radiation, EUV, X-rays, electrons and ions.